Segregation and dissolution behavior of Mg alloyed with Ca and Al are studied by performing density functional theory calculations considering an extensive set of surface structures and compositions. Combining ab initio surface science approaches with cluster expansion for ordered surface structures we construct surface phase diagrams for these alloys. We utilize these diagrams to study segregation phenomena and chemical trends for surfaces in contact with a dry environment or with an aqueous electrolyte. We show that the presence of water dramatically impacts the stability and chemical composition of the considered metallic surfaces. We furthermore find that the two alloying elements behave qualitatively different: Whereas Ca strongly segregates to the surface and becomes dissolved upon exposure of the surface to water, Al shows an antisegregation behavior, i.e., it remains in Mg bulk. These findings provide an explanation for the experimentally observed increase/decrease in corrosion rates when alloying Mg with Al/Ca.

The kinetics of vacancies in materials plays a significant role in determining their physical properties. In this work, we investigate diffusion of vacancies in $\alpha$-iron using the kinetic activation-relaxation technique, an off-lattice kinetic Monte Carlo method with on-the-fly catalog building based on the activation-relaxation technique nouveau coupled with an embedded atom method potential. We focus on the evolution of one to eight vacancies to provide a detailed picture of the energy landscape, overall kinetics, and diffusion mechanisms associated with these defects. We show formation energies, activation barriers for the ground state of all eight systems, and migration barriers for the diffuse systems. This study points to an unsuspected dynamic richness, even for this simple system, that can only be discovered through comprehensive and systematic approaches such as the kinetic activation-relaxation technique. The complex energetic environment controlling the kinetics of small vacancy clusters, we find here, demonstrates that simple rules are not sufficient to develop a robust approach to predictive control and prevention of damage processes associated with vacancy clusters in structural metals.

Breaking time-reversal symmetry in a Dirac semimetal ${\mathrm{Cd}}_3{\mathrm{As}}_2$ through doping with magnetic ions or by the magnetic proximity effect is expected to cause a transition to other topological phases (such as a Weyl semimetal). To this end, we investigate the possibility of proximity-induced ferromagnetic ordering in epitaxial Dirac semimetal (${\mathrm{Cd}}_3{\mathrm{As}}_2$)/ferromagnetic semiconductor (${\mathrm{Ga}}_{1-x}{\mathrm{Mn}}_x\mathrm{Sb}$) heterostructures grown by molecular beam epitaxy. We report the comprehensive characterization of these heterostructures using structural probes (atomic force microscopy, x-ray diffraction, scanning transmission electron microscopy), angle-resolved photoemission spectroscopy, electrical magnetotransport, magnetometry, and polarized neutron reflectometry. Measurements of the magnetoresistance and Hall effect in the temperature range 2–20 K show signatures that could be consistent with either a proximity effect or spin-dependent scattering of charge carriers in the ${\mathrm{Cd}}_3{\mathrm{As}}_2$ channel. Polarized neutron reflectometry sets constraints on the interpretation of the magnetotransport studies by showing that (at least for temperatures above 6 K) any induced magnetization in the ${\mathrm{Cd}}_3{\mathrm{As}}_2$ itself must be relatively small $(<14\mathrm{emu}/{\mathrm{cm}}^3)$.

In glasses under mechanical load, intrinsic spatial inhomogeneities at specific locations in the sample may cause shear banding. This allows us to initiate mechanical failure in a controlled manner. We perform molecular dynamics simulations to investigate inhomogeneous glass states under shear, using two different protocols to obtain these spatial inhomogeneities, viz., (i) by applying a temperature pulse, and (ii) by generating regions with a different degree of annealing via the swap Monte Carlo technique. In both cases, we find that shear banding is associated with a subtle interplay between stochasticity and local potential energy.

Traditional approaches to identify ion-transport pathways often presume an equal probability of occupying all hopping sites and focus entirely on finding the lowest migration barrier channels between them. Although this strategy has been applied successfully to solid-state Li battery materials, which historically have mostly been ordered frameworks, in the emerging class of disordered electrode materials some Li sites can be significantly more stable than others due to a varied distribution of transition metal (TM) environments. Using kinetic Monte Carlo simulations, we show that in such cation-disordered compounds only a fraction of the Li sites connected by the so-called low-barrier “0-TM” channels actually participate in Li diffusion. The Li-diffusion behavior through these sites, which are determined primarily by the voltage applied during Li extraction, can be captured using an effective migration barrier larger than that of the 0-TM barrier itself. The suppressed percolation due to cation disorder can decrease the ionic diffusion coefficient at room temperature by over two orders of magnitude.

III-V ternary nanowires and nanowire heterostructures offer almost unlimited possibilities for the band-gap design and can be integrated with Si electronic platform. Most of such nanowires are grown by the vapor-liquid-solid method. The presence of a catalyst droplet, which is a quaternary liquid alloy for Au-catalyzed III-V ternary nanowires, significantly complicates the analysis of the nanowire composition versus the vapor and liquid compositions. Herein, we develop a growth theory of III-V ternary nanowires and obtain the stationary liquid-solid distribution in a rather general form. It is shown that the liquid-solid distribution is close to equilibrium, or nucleation limited, for nanowires based on group III intermix, and becomes kinetically controlled for nanowires based on group V intermix. These principally different behaviors are due to extremely low concentrations of highly volatile group V elements in a droplet, leading to group V limited liquid-solid growth. We also consider the vapor-solid distributions of III-V ternary nanowires and demonstrate that they are controlled by the material influxes into the droplet, and almost independent of the liquid composition in many cases, at least for ternaries based on group III intermix. The examples of the liquid-solid and vapor-solid distributions are given for different material systems including Au-catalyzed InGaAs and AlGaAs nanowires, and self-catalyzed InPAs, GaSbAs, and AlSbAs nanowires, which fit very well the available experimental data.

We present and characterize a new member of the $R{\mathrm{Cd}}_3{\mathrm{P}}_3$ ($R=\text{rare earth}$) family of materials, ${\mathrm{NdCd}}_3{\mathrm{P}}_3$, which possesses ${\mathrm{Nd}}^{3+}$ cations arranged on well-separated triangular lattice layers. Magnetic susceptibility and heat capacity measurements demonstrate a likely $S_{\mathrm{eff}}=1/2$ ground state, and also reveal the formation of long-range antiferromagnetic order at $T_N=0.34\mathrm{K}$. Via measurements of magnetization, heat capacity, and electrical resistivity, we characterize the electronic properties of ${\mathrm{NdCd}}_3{\mathrm{P}}_3$ and compare results to density functional theory calculations.

Certain intrinsic point defects in silicon carbide are promising quantum systems with efficient spin-photon interface. Despite carbon vacancy in silicon carbide being an elementary and relatively abundant intrinsic defect, no optical signal has been reported associated with it. Here, we revisit the positively charged carbon vacancy defects in the 4H polytype of silicon carbide (4H-SiC) by means of ab initio calculations. We unveil the origin of thermal averaging observed in the EI5 electron spin resonance center associated with the so-called k-site configuration of carbon vacancy as phonon excitations in the strongly coupled electron-phonon system. We further find that the excited state is optically active for the so-called h-site configuration of carbon vacancy, with zero-phonon line at $0.65\mathrm{eV}$. This defect shows a strong emission with a 9.3 ns optical lifetime. Moreover, the deteriorating effects of phonons on its optical properties are relatively weak, evinced by the calculated quantum efficiency and Debye-Waller factor of 27% and 20%, respectively. Based on these findings, we propose this defect as an exotic paramagnetic near-infrared emitter in the IR-B region.

Photoinduced structural dynamics of rare-earth nickelate thin films, ${\mathrm{NdNiO}}_3$ and ${\mathrm{SmNiO}}_3$, grown on (001) oriented ${\mathrm{SrTiO}}_3$ were studied using time-resolved x-ray diffraction. The evolution of the (002) Bragg peak was tracked following laser excitation. The recovery pathways were found to be strongly dependent on laser fluence for ${\mathrm{NdNiO}}_3$ and distinct for the two rare-earth nickelates. The recovery of the (002) peak shifts was modeled using a one-dimensional thermal diffusion model which showed that the recovery processes are nonthermal at high fluences. For ${\mathrm{NdNiO}}_3$, the timescales for the recovery of the (002) peak shift were found to be closely related to Ni magnetism recovery, potentially indicating magnetostructural coupling. Moreover, the evolution of integrated intensity and full width at half maximum points towards the presence of a structural phase separation during recovery. Our studies highlight structural recovery pathways in nickelates by comparing the distinct responses of the photoinduced metal-insulator transition in ${\mathrm{NdNiO}}_3$ and ${\mathrm{SmNiO}}_3$.

Tungsten is a promising candidate for the plasma-facing material in fusion energy facilities, however, the low-energy, high-flux hydrogen plasma causes severe blistering in tungsten, which gives rise to safety concerns. By far, the formation mechanism of intragranular hydrogen blisters is still unclear. Large-scale atomistic simulations are crucial for improving the understanding, however, the available empirical interatomic potentials are mostly defective in predicting the formation of hydrogen self-clusters in tungsten, thus may lead to wrong blister formation mechanisms. In this work, we develop a machine-learning potential, DP-WH, for the tungsten-hydrogen binary system based on the Deep Potential method. We demonstrate that the DP-WH potential is able to describe, as accurately as ab initio calculations, the basic properties of bcc tungsten, the solute hydrogen properties in tungsten, adsorption and migration of hydrogen on tungsten free surfaces, interactions between hydrogen atoms and vacancy, dislocations, the interaction between neighboring interstitial hydrogen atoms, and the formation energy of H self-clusters. By using DP-WH, we perform nanosecond-long molecular dynamics simulations and report the formation of planar self-cluster of tetrahedral-interstitial-site hydrogen atoms normal to $\left\{001\right\}$ tungsten planes at a hydrogen concentration of $\approx 10$ at.%. This form of the H self-cluster is highly possible to be the early nucleates of the crack-shaped H blisters observed in recent experiments. The DP-WH is thus proven as a good candidate potential for the atomistic simulations to unveil the formation mechanisms of the intragranular hydrogen blisters in tungsten under the relevant working conditions.

Rare-earth iron garnets (ReIGs) are attracting considerable attention because they could enable many intriguing spintronic phenomena. Material choices of these garnets are, however, limited to rather simple compositions, such as ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$, ${\mathrm{Tm}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$, and ${\mathrm{Gd}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$, among many others. The magnetic properties of these garnets, including saturation magnetization, anisotropy, and damping parameter, are accordingly limited within a narrow parameter space. In this study, we will first grow a series of substituted ReIGs of composition ${\mathrm{Gd}}_{3-x}{\mathrm{Ho}}_x{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ $(x_{\mathrm{Ho}}=0-3)$. Subsequently, a continuous tuning of magnetic parameters, including the net saturation magnetization $\left(M_S\right)$, the magnetization compensation temperature $\left(T_M\right)$, and the damping parameter $\left(\alpha \right)$, are also demonstrated through linearly changing the Ho concentration. The present method could be useful for designing garnets with desired material parameters, which could be incorporated for enabling unique spintronic devices based on compensated ReIGs.

Resistive memory based on two-dimensional (2D) tungsten disulfide (${\mathrm{WS}}_2$), molybdenum disulfide (${\mathrm{MoS}}_2$), and hexagonal boron nitride (h-BN) materials is studied via experiments and simulations. The influence of the active layer thicknesses is discussed, and the thickness with the best on/off ratio is found for 2D resistive random-access memory (RRAM). This work reveals fundamental differences between a 2D RRAM and conventional oxide RRAM. Furthermore, the physical parameters extracted using the kinetic Monte Carlo (KMC) model indicate that 2D materials have a lower diffusion activation energy along the vertical direction, where a smaller bias voltage and a shorter switching time can be achieved. The diffusion activation energy from the chemical vapor deposition (CVD)-grown sample is much lower than for mechanically exfoliated samples. The results suggest that ${\mathrm{MoS}}_2$ has the fastest switching speed among the three considered 2D materials.

The topotactic intercalation of transition-metal dichalcogenides with atomic or molecular ions acts as an efficient knob to tune the electronic ground state of the host compound. A representative material in this sense is $1T\text{−}{\mathrm{TiSe}}_2$, where the electric-field-controlled intercalations of lithium or hydrogen trigger superconductivity coexisting with the charge-density-wave phase. Here, we use the nuclear magnetic moments of the intercalants in hydrogen-intercalated $1T\text{−}{\mathrm{TiSe}}_2$ as local probes for nuclear magnetic resonance experiments. We argue that fluctuating mesoscopic-sized domains nucleate already at temperatures higher than the bulk critical temperature to the charge-density-wave phase and display cluster-glass-like dynamics in the MHz range tracked by ${}^1\mathrm{H}$ nuclear moments. Additionally, we observe a well-defined independent dynamical process at lower temperatures that we associate with the intrinsic properties of the charge-density-wave state. In particular, we ascribe the low-temperature phenomenology to the collective phasonlike motion of the charge-density wave being hindered by structural defects and chemical impurities and resulting in a localized oscillating motion.

We determine the impact of anharmonic thermal vibrations on the fundamental band gap of ${\mathrm{CsPbBr}}_3$, a prototypical model system for the broader class of halide perovskite semiconductors. Through first-principles molecular dynamics and stochastic calculations, we find that anharmonic fluctuations are a key effect in the electronic structure of these materials. We present experimental and theoretical evidence that important characteristics, such as a mildly changing band-gap value across a temperature range that includes phase transitions cannot be explained by harmonic phonons thermally perturbing an average crystal structure and symmetry. Instead, the thermal characteristics of the electronic structure are microscopically connected to anharmonic vibrational contributions to the band gap that reach a fairly large magnitude of 450 meV at 425 K.

According to classical metal physics theory, the substitutional solute atom in dilute metal solid solution occupies the high-symmetry lattice site without destroying the point group symmetry of the host lattice. However, our recent first-principles calculations showed that the substitutional solute atom Mo occupies the low-symmetry off-center position in dilute Ti solid solutions such that breaking spontaneously the point group symmetry of the hexagonal-close-packed host lattice [Q. M. Hu and R. Yang, Acta Mater. 197, 91 (2020)] resulted from the Jahn-Teller splitting of the degenerated $d$ orbitals of Mo at the Fermi level. Whether the spontaneous symmetry breaking is general for some other solid solutions and robust (e.g., to the temperature effect) remains an open question. In the present work, we investigated the site occupation of substitutional atom $X$ in dilute Ti-$X$ solid solution with $X$ being all the $3d$ metal elements from V to Zn. Among the considered solute atoms, Cr, Mn, Fe, and Co are dynamically and energetically stable to occupy the low-symmetry off-center site such that the spontaneous symmetry breaking occurs, whereas V, Ni, Cu, and Zn prefer to stay at the high-symmetry lattice site. We demonstrated that the off-center occupation is robust against the supercell size adopted in the first-principles calculation and the thermal volume expansion.

Using low temperature scanning tunneling microscopy (STM), spectroscopy (STS), and precise $dI/dV$ mapping in combination with density-functional theory-parametrized semiempirical calculations, we report and discuss the origin of tunneling electronic excitations that occur along a seven-carbon-atom-wide armchair graphene nanoribbon (7-aGNR) physisorbed on Ag(111) as compared to a reference on Au(111) surface. On both surfaces, on-surface synthesized 7-aGNRs of variable lengths are selectively chosen for performing the STM and STS ($dI/dV$) excitation mapping along a truly isolated molecule. For exactly the same 7-aGNR molecule length, the difference in work functions between Ag(111) and Au(111) generates different edge states electronic configuration that result in a curved molecular conformation on Ag(111) and a strictly flat one on Au(111). At the interface between the 7-aGNR and Ag(111), this curved conformation produces an additional set of quantum-box-like tunneling resonances that partially mix with the intrinsic 7-aGNR tunneling excitations.

In spinel-type nickelate ${\mathrm{NiRh}}_2{\mathrm{O}}_4$, magnetic ordering is observed upon the sample synthesized via kinetically controlled low-temperature solid-state metathesis, as opposed to previously reported samples obtained through conventional solid-state reaction. Our findings are based on a combination of bulk susceptibility and specific heat measurements that disclose a Néel transition temperature of $T_{\mathrm{N}}=45$ K in this material, which might feature spin-orbit entanglement in the tetragonally coordinated $d^8$ Mott insulators. The emergence of magnetic ordering upon alteration of the synthesis route indicates that the suppression of magnetic ordering in the previous sample was rooted in the cation mixing assisted by the entropy gain that results from high-temperature reactions. Furthermore, the $J_{\mathrm{eff}}=0$ physics, instead of solely the spin-only $S=1$, describes the observed enhancement of effective magnetic moment well. Overseeing all observations and speculations, we propose that the possible mechanism responsible for the emergent magnetic orderings in ${\mathrm{NiRh}}_2{\mathrm{O}}_4$ is the condensation of the $J_{\mathrm{eff}}=1$ exciton, driven by the interplay of the tetragonal crystal field and superexchange interactions.

Ferroelectric diodes can generate a polarization-controlled bidirectional photoresponse to simulate inhibition and promotion behaviors in the artificial neuromorphic system with fast speed, high energy efficiency, and nonvolatility. However, the existing ferroelectric diodes based on ferroelectric oxides suffer from a weak bidirectional photoresponse (below 1 mA/W), difficult miniaturization, and a large response photon energy (over 3 eV). Here, we design a series of van der Waals $\alpha \text{−}{\mathrm{In}}_2{\mathrm{Se}}_3\text{/}\mathrm{Nb}{X}_2$ ($X$ = S, Se, and Te) ferroelectric diodes with bidirectional photoresponse by using ab initio quantum transport simulation. These devices show a maximum bidirectional photoresponse of 30 (−19) mA/W and a minimum response photon energy of 1.3 eV at the monolayer thickness. Our work shows advanced optoelectronic applications of the van der Waals ferroelectric diodes in the future artificial neuromorphic system.